A screw compressor in which a gas is caused to expand against a rotor has a housing in which a pair of intermeshing rotors turn. At one end of the housing there is an inlet or intake port and at the other end an outlet or output port. The rotors are formed with meshing helical formations having the general form of multiple screw threads.
As the rotors turn in opposite senses they form a plurality of helical chambers or channels. Each chamber is defined at one side by an axial end of the housing and at the other side by a region of contact between two meshing helical formations. This region moves axially from inlet end to outlet end, compressing air ahead of it against the outlet end of the compressor and drawing air in through the inlet. When the helical chamber so formed is brought around to the outlet port, the compressed gas is expelled.
The efficiency of such compressors or expansion machines depends mainly upon the tightness of the seal between the rotors and the housing. The rotors form with each other a helical line of seal that must be as tight as possible in order for any efficiency to be obtained. This, of course, requires that the rotors be machined very carefully. Usually the so-called main rotor is formed with four helical threadlike formations which are semicircular in section. The secondary rotor is formed with six mating helical semicircular recesses, and the two parallel rotors are of like outer diameter. Thus one of these rotors wipes the other so that the flanks of the ribs of the main rotor must be very steep adjacent their root, and the flanks of the ribs of the secondary rotor must be very steep adjacent their tops. Machining of these surfaces is very difficult as a very small clearance angle is required which makes the cutting operation very slow and rapidly dulls the cutting tool. The tools must be reground frequently, which greatly increases the cost of the machining and hence of the compressor.
As a result of the cost of the machining to make the rotors for such screw machines usable as compressors, pumps, or expansion machines, they are frequently considered too expensive even though the particular advantages of such devices--small size, long service life--would make them ideal for many tasks.